It has become more common to power LEDs and LED circuits using AC voltage, and in particular AC line voltage. The LEDs or LED circuits are typically integrated into a lighting system, device or lamp, and may be configured in a manner in which LEDs alternate turning on and off with the current. For example LEDs may be configured in an anti-parallel relationship or may be configured in a bridge or unbalanced bridge configuration as shown in Lynk Labs U.S. Pat. Nos. 7,489,086 and 8,179,055.
Alternatively, and more typically, LEDs and LED circuits driven with an input of AC power from an AC power source are provided with voltage by a full or half wave rectifier placed between the LEDs, or LED circuits, and the AC power source as seen for example in Lynk Labs U.S. Patent Publication No. 2012/0293083. FIG. 1 generally shows an example of a known linear step drive topology. FIG. 1, for example, shows a series string of LEDs forming a single LED circuit, with groups of LEDs in the circuit being connected in parallel with distinct switches. Series string or LED string should be understood in the art to mean two or more LEDs connected in series with each other, i.e. a series circuit of multiple LEDs or in some cases LED circuits. In such configurations, as the provided voltage increases, the switches will begin to open causing more LEDs to turn on to match the voltage—for example, in FIG. 1 once the provided forward voltage is enough for the LEDs in the first segment to turn on the first switch in parallel with the first segment will open causing current to flow through those LEDs causing light emission, once the forward voltage is enough to turn on the first and second segment of LEDs, the second switch will open causing current to flow through the second segment of LEDs along with the first segment of LEDs thereby following and closer matching the input voltage level.
Rather than use the configuration discussed above, in order to attempt to address flicker and protect the LEDs, some systems and devices operate in a similar manner to a linear step drive. Rather than have a single series string with multiple groups divided by parallel bypass switches, these system and devices may have multiple series string of LEDs each having different numbers of LEDs with the series strings being connected in parallel. Once the forward operating voltage is enough to drive the first series string having a set number of LEDs, the first series string will be switched on and provided with voltage. Once the forward operating voltage is large enough to drive the second series string, the first series string may be switched off and the second series string switched on alone or along with the first series string, and so on.
Linear step drive topologies like that shown in FIG. 1 or similar configurations have been shown to have a satisfactory power factor and very low overall total harmonic distortion, however they, like directly driven AC LED circuits, have two major problems that must be addressed—they do not completely solve the flicker issue, and they create a near constant changing level of light flux emitted by the device as different numbers of LEDs turn on and off.
Many of the known prior art systems fail to reduce or even eliminate flicker in response to an AC voltage source, and/or for the period where the AC voltage is not high enough to drive any LEDs or LED circuits in the drive system, i.e. at the beginning and end of each half cycle of input AC or rectified AC voltage. As the voltage alternates, whether it is provided directly to an LED circuit or rectified first, as the voltage approaches and crosses zero, there will reach a point where the provided voltage is less than the forward operating voltage of any LEDs or LED circuits in the device. When the input voltage drops below the lowest forward operating voltage required to drive any LEDs or LED circuits in the device or system, all the LEDs will effectively be turned off, creating a brief moment where the system or device emits no light. In this sense flicker is created as the system or device stops emitting light for a brief moment, causing the light to turn off before the provided AC voltage is back above the lowest operable forward operating voltage in the device.
Though flicker in LEDs may be imperceptible to individuals above the threshold above a certain frequency, like for example approximately 70 Hz, and LEDs will typically operate at approximately between 100 Hz or 120 Hz in countries around the world, studies have shown that animals and some humans may be effected at this range, and stroboscopic effects may be visible when moving objects are illuminated by a system or device at a second, higher frequency, like for example, 120 Hz or higher. In order to prevent problems associated with flicker, it has been found that a modulation rate of over a certain frequency, like for example 200 Hz or higher is required. The present systems and devices known in the art only provide this using electronic transformers or the like.
In order to address the issue associated with flicker, there have been apparatuses developed which attempt to provide some level of power to LEDs during the periods at the beginning and end of each half cycle. For example, systems have been developed which include a switch controlled capacitor or multiple capacitors which may be used to store power during a peak current of each half cycle of an input voltage, and discharge that power to an entire or a portion of a linear step drive circuit at the beginning, end and in between half cycles. While this configuration may help alleviate some of the issues associated with flicker, unless very large levels of capacitance are provided, the power stored is usually less than that required to maintain the level of voltage and current necessary to fill the entire gap from the end of one half cycle through the beginning of the next half cycle, particularly since the proposed apparatuses to date do not provide any control for when and/or how the discharge of the capacitor will occur in response to the AC input. Control is only provided to control the charging of the capacitor.
Furthermore, the combination of a switch controlled capacitor and a linear step circuit do nothing to alleviate the issues related to the near constant changing level of light flux emitting from the apparatus as it is still a linear step drive.
In linear step drives or similar circuits, as the voltage increases, the number of LEDs turned on in series likewise increases to increase the forward operating voltage to match the input voltage provided by the AC voltage source. Conversely, as the voltage decreases in magnitude and approaches zero at the end of the half cycle, the number of LEDs turned on in series will decrease to match the forward operating voltage to the decreasing input voltage. As the voltage builds towards it peak magnitude, the amount of light provided by the lighting systems or device will increase as more LEDs in series and/or LED circuits are turned on in order to increase the forward operating voltage and match the input voltage. Once the voltage reaches its peak magnitude and begins to decrease, fewer LEDs and/or LED circuits will be turned on in order to insure that the forward operating voltage is not greater than the provided input voltage and insure that at least some of the LEDs are on and emitting light. As LEDs and/or LED circuits are turned on and off in such configurations, the amount of light emitted by the system or device increases and decreases, causing a near constant change in the light flux of the entire device. The total power dissipation likewise is in constant flux, reflecting the change in flux as LEDs are turned on and off in different numbers.
The present invention is provided to solve these and other issues.